std::ratio provides convenience typedefs for metric prefixes (centi, deci, deca, hecto).
yocto std::ratio<1, 1000000000000000000000000>, if std::intmax_t can represent the denominator
zepto std::ratio<1, 1000000000000000000000>, if std::intmax_t can represent the denominator
atto std::ratio<1, 1000000000000000000>
femto std::ratio<1, 1000000000000000>
pico std::ratio<1, 1000000000000>
nano std::ratio<1, 1000000000>
micro std::ratio<1, 1000000>
milli std::ratio<1, 1000>
centi std::ratio<1, 100>
deci std::ratio<1, 10>
deca std::ratio<10, 1>
hecto std::ratio<100, 1>
kilo std::ratio<1000, 1>
mega std::ratio<1000000, 1>
giga std::ratio<1000000000, 1>
tera std::ratio<1000000000000, 1>
peta std::ratio<1000000000000000, 1>
exa std::ratio<1000000000000000000, 1>
zetta std::ratio<1000000000000000000000, 1>, if std::intmax_t can represent the numerator
yotta std::ratio<1000000000000000000000000, 1>, if std::intmax_t can represent the numerator
What's missing? Well... the unit ratio std::ratio<1,1>.
I am aware that there is no official metric prefix name for the unit, but that does not mean that it does not exist.
Nowhere in [ratio.si], the unit prefix is mentioned. So I wonder: what would be the most paradigmatic way to work with a 'unit' ratio? For example, when duration_cast-ing to whole seconds.
what would be the most paradigmatic way to work with a 'unit' ratio?
The most pragmatic way to work with a unit ratio is to not use it.
It's a bit like asking what is the best way to multiply by 1. You don't.
For example, when duration_cast-ing to whole seconds.
You would write std::chrono::duration_cast<std::chrono::seconds>.
std::ratio<1,1> has no name because you never need a name for it. For example std::duration has already a default period of std::ratio<1,1>.
If you still want to give it a name you can do so:
using unit_ratio = std::ratio<1>;